Conductive paint formulations with very low electrical impedance in the Z-direction containing a metal carbide

ABSTRACT

One or more metallic carbides having the general structure MC in which M is selected from the group consisting of Titanium (Ti), Zirconium (Zr), Niobium (Nb), Hafnium (HF), Tantalum (Ta) and Tungsten (W) are mixed with standard paints to produce a paint which will provide a coating that has a very low electrical impedance in a direction orthogonal to the plane of the coating.

United States Patent Neumann et a1.

CONDUCTIVE PAINT FORMULATIONS WITH VERY LOW ELECTRICAL IMPEDANCE IN THEZ-DIRECTION CONTAINING A METAL CARBIDE Inventors: Edward W. Neumann;Francis J.

Rahemba; Stanley Scheinberg, all of Poughkeepsie, N.Y.

Assignee: International Business Machines Corporation, Armonk, N.Y.

Filed: June 17, 1974 Appl. No.: 479,975

U.S. Cl. 260/18 PN; 117/121; 117/126; 260/29.6 MM; 260/29.7 M; 260/37 M;260/42.2 Z

Int. C1. C23F 5/02 Field of Search... 260/18 PN, 37 M, 29.6 MM, 260/29.7M, 42.2 Z; 117/121, 126

References Cited UNITED STATES PATENTS 4/1966 Trevoy 252/516 Oct. 14,1975 3,380,935 4/1968 Ring 252/511 3,404,031 10/1968 Clayton et a1.117/226 3,563,916 2/1971 Takashina et a1 1 17/226 3,676,212 7/1972Mishler 117/226 3,746,662 7/ 1973 Adelman 260/37 3,783,021 1/1974 York117/226 3,788,997 1] 1974 MacKenzie 252/516 Primary Examiner-Eugene C.Rzucidlo Attorney, Agent, or Firm-Edward S. Gershuny 4 Claims, NoDrawings CONDUCTIVE PAINT FORMULATIONS WITH VERY LOW ELECTRICALIMPEDANCE IN THE Z-DIRECTIION CONTAINING A METAL CARBIDE BACKGROUND OFTHE INVENTION The invention relates to compositions which can be used toprovide a coating which has low electrical impedance. More particularly,the invention relates to compositions of paints which, when dry, willprovide a coating which has a very low electrical impedance in adirection orthogonal to the plane of the coating (its Z- direction").

When painting various surfaces such as, for example, the exterior panelsof electronic data processing machines, it is often necessary ordesirable to provide a means for establishing a path of low electricalimpedance through the paint film. In the prior art, this is generallyaccomplished in either of two ways.

The first technique utilizes a conductive material which is attachedthrough or around the painted substrate to provide a contact point oneach side thereof. One example of this is the common use of so-calledrub strips on, for example, the painted doors which cover various typesof electrical equipment. An advantage of this approach is that it can beutilized when the substrate is a non-conducting material. Onedisadvantage is that the attachment of the conductive pieces generallyrequires an additional step in the manufacture and therefore involvesincreased labor cost. Another disadvantage is that the contact pointsestablished by this technique are generally rather small in relation tothe size of the painted substrate; therefore requiring, in somesituations, an extra amount of care in locating the electrical contacts.

When the substrate is, itself, an electrically conductive material,electrical through-connections can be achieved by painting it with anelectrically conductive paint. Assuming that the substrate is to bepainted anyway, no additional manufacturing step is required. Also,contact can be established over a relatively large area.

In accordance with one prior art technique, a conductive paint is madeby adding, to any ofa large number of known paint formulations aconductive filler. The two fillers most commonly used are silver andgraphitic carbon. As between the two, a primary advantage of graphiticcarbon is that is is significantly less expensive than silver. Also, itsdensity is less than that of silver, and it therefore has a lessertendency to settle out of the paint mixture. The primary advantage ofsilver is that it is a better conductor than graphitic carbon. When asubstrate is painted with a commercially available conductive paint toproduce a coating of approximately 1 to 2 mils thickness, resistivity inthe Z- direction will typically be approximately 0.1 ohms for paintusing a silver filler, and 600 ohms for paint using a carbon filler.(Although silver filled paints can have excellent conductivity, itshould be noted that they are extremely expensive, costing as much asabout $400 per gallon.)

Another filler which has been used is copper. However, copper has thedisadvantage that it is subject to oxidation and the oxides are notconductive.

An object of this invention is to provide a paint having a very lowelectrical impedance in its z-direction.

A further object of the invention is to provide such a paint which willbe economical to use when painting large surfaces.

Another object of the invention is to provide such a conductive paintwhich, when dry, will present an appearance that is estheticallypleasing.

Other objects of the invention are to provide such a conductive paintwhich, when dry, will have good characteristics of adhesion and impactresistance.

BRIEF DESCRIPTION OF THE INVENTION In accordance with a preferredembodiment of the invention, the above and other objects areaccomplished by providing a paint into which one or more metalliccarbides are mixed. The metallic carbides have the general structure MCin which M is selected from the group consisting of Titanium (Ti),Zirconium (Zr), Niobium (Nb), Hafnium (HF), Tantalum (Ta), and Tungsten(W). Although carbide particles of a size up to about 44 microns may beused to advantage, the preferred range of particle size is approximatelyI to 5 microns. The concentration of carbide particles in the paint willpreferably be in the range of 10 to 40% by dry weight.

The above and other objects, features and advantages of this inventionwill be apparent from the following description of preferred embodimentsthereof.

DETAILED DESCRIPTION Method of Testing Before describing specificexamples of paint formulations, it will be appropriate to describe themanner in which the various formulations were tested.

Coating formulations wereprepared and applied to phosphatized steelpanels such as those used by the paint industry for the evaluation ofpaints. These panels are approximately 3 inches wide X 5 inches long and0.027 inches thick. The formulations described in the following exampleswere applied to the above mentioned steel panels and performancescreening tests were used to determine the electrical contact impedance,the adhesion of the dried or baked film to the steel substrate, theimpact resistance of the dried or baked film and its esthetic qualities.

The electrical contact impedance was measured by placing one lead of aDC. ohmeter on the non-painted underside of the steel panel and placingthe other lead from the ohmeter on the paintedsurface of the same panel.The resistance of the paint film can then be read directly from theohmeter in ohms.

The adhesion of the coating to the steelpanel is measured by a tapetest. This tes: is performed by scoring six parallel lines about 4; inchapart and 1 inch long on the coated steel panel. The scorings mustpenetrate the coating to the steel substrate. This may be accomplishedby using a pointed scalpel blade or sharp knife. Six more scorings aremade similarly but at to the first set of six scorings and superimposedupon the first set such that a cross-hatch pattern results in whichthere is a grid 25 squares, each approximately 4; X /8 inch. A piece oftransparent cellophane tape 1 inch wide by about 6 inches long is firmlypressed down on the previously described grid and then pulled offbriskly at an angle of approximately 90 to the steel panel. The adhesiontest is said to have been passed" if none of the paint is removed.

The impact test is a measure of the toughness" and resiliency of a film.The test is performed using a commercially available impact tester. Aknown weight with a spherical head is dropped head down from a measuredheight. If the coating on the steel panel is fractured or in any waydamaged it is said to have failed the test. The results are stated belowin inch-lbs.

Tests on the esthetic qualities of painted panels were made bysubjective human observation and were primarily directed to the visualappearance and tactile feel of each painted panel.

EXAMPLES OF PAINT FORMULATIONS Each of the following formulations wasprepared and a steel panel was spray painted, then baked and tested asdescribed above. In each case the coating of paint was approximately 1to 2 mils thick. All proportions given below are by weight with theexception of the solvent, which is by volume. As is well known in theart, the amount of solvent which should be used for a given applicationwill depend upon the manner in which the paint is to be applied (forexample, by brushing or by spraying) and the viscosity desired. In eachof the following examples, the solvent used was a mixture of (by volume)20 parts toluene, parts methyl ethyl ketone (MEK), 10 parts methylisobutyl ketone (MIBK), 10 parts benzene and 25 parts ethyl acetate.

In each of the following examples, the impact resistance of the paintedpanel was measured as being in excess of 50 inch-lbs., and each paintedpanel passed the adhesion test.

EXAMPLE I 50 parts ERL 2795 Epoxy resin 50 parts Versamid I 40 parts TiC(325 mesh) parts graphite (A-99) 60 parts solvent viscosity 110 cpimpedance: less than 1 ohm EXAMPLE 2 50 parts ERL 2795 Epoxy resin 50parts Versamid 115 40 parts ZrC (l5 micron) 20 parts graphite (A-99) 60parts solvent viscosity 90 cp impedance: less than 1 ohm EXAMPLE 3 50parts ERL 2795 Epoxy resin 50 parts Versamid l 15 40 parts TaC (l-5micron) 20 parts graphite (A-99) 60 parts solvent viscosity 1 l0 cpimpedance: 500-1500 ohms EXAMPLE 4 50 parts ERL 2795 Epoxy resin 50parts Versamid I15 40 parts WC (l5 micron) 20 parts graphite (A-99) 70parts solvent viscosity 1 l0 cp impedance: more than 5000 ohms EXAMPLE 550 parts ERL 2795 Epoxy resin 50 parts Versamid 1 15 60 parts graphite(A-99) 100 parts solvent viscosity 80 cp impedance: 2200-2600 ohms Inthe formulations described in the above five examples, the ERL 2795epoxy resin is a product of the Union Carbide Corporation, the Versamidis a product of General Mills Chemical Corporation and the graphite is aproduct of Asbury Graphite Mills.

It was found that the higher density carbides such as TaC and WC producefilms having fairly high contact resistances. This is not due to theinherent resistivity of these carbides but rather to the fact that,because of their high densities, they settle more rapidly in the spraycan: thus less of these carbides appear in the coating on the steelpanel.

Example 5 shows that if graphite alone is used as a conductive filler,the contact resistance is about 2200-2600 ohms. The replacement of partof the graphite component in the formulation, with TiC (Example or ZrC(Example 2) reduces the contact resistance by more than three orders ofmagnitude.

It was also found that, the 325 TiC used in Example 1, while it has goodphysical and electrical properties, is very gritty because of itscoarseness (up to 44 microns). On the other hand, the l-5 micron ZrCused in Example 2 provides a much smoother and esthetically acceptabletexture.

In order to determine the effective range of loading of conductive metalcarbides, the following examples were tested for electrical contactresistance when steel panels were sprayed and baked.

Example 6 Contact Resistance 300-600 ohms Example 7 Glidden l45l-lconductive paint 2-5 ohms 10% by day weight TiC 325 mesh Example 8Contact Resistance Glidden l45l-l conductive paint less than I ohm 20%by day weight TiC 325 mesh The Glidden conductive paint cited inexamples 6, 7, and 8 is a commercially available paint designated 1451-1by The Glidden-Durkee Division of S.C.M. Corporation.

Two formulations similar to Example I were prepared in which the 325mesh (up to 44 micron) TiC was replaced by l-5 micron TiC (and bysubmicron TiC) and steel panels were spray painted as before. In bothcases, the contact resistance was less than one ohm, the impactresistance was inch-lbs. and the coating passed the adhesion test.

A commercially available water reducible conductive paint (made by TheGlidden-Durkee Division, S.C.M. Corp. designated Black AqualureConductive Coating") was modified by the addition of l-5 micron TiC.

The additions by weight are tabulated below along with the electricalcontact resistance.

The intended ultimate use of the conductive coating governs the amountof MC which should be added to obtain a low impedance ground path.

For example, if there is a requirement that only small portions of aco-member be in contact with the painted surface, then the loading wouldnecessarily need to be relatively high; i.e., 20-40% of MC. If however,there are massive contact areas such as a computer cover in contact witha computer frame, then -15% loading of MC would be adequate.

It should also be noted that the particle size range will dictate thetexture of the dried or baked coating.

While the invention has been particularly shown and described withreference to preferred embodiments thereof, it will be understood bythose skilled in the art that the above and other changes in form anddetails may be made therein without departing from the spirit and scopeof the invention.

What is claimed is:

l. A conductive paint formulation including:

10 to 40% by dry weight metallic carbide ofthe structure MC; wherein Mis selected from the group consisting of titanium, zirconium, niobium,hafnium, tantalum and tungsten; and wherein the car bide particles areup to 44 microns in size.

2. The conductive paint formulation of claim 1 wherein: theconcentration of carbide particles is between 10 and 30% by weight.

3. The conductive paint formulation of claim 2 wherein: the carbideparticles consist essentially of particles within the range of size of 1to 5 microns.

4. The conductive paint formulation of claim 3 wherein: theconcentration of carbide particles is within the range of 10 to 15% byweight.

1. A CONDUCTIVE PAINT FORMULATION INCLUDING: 10 TO 40% BY DRY WEIGHTMETALLIC CARBIDE OF THE STRUCTURE MC, WHEREIN M IS SELECTED FROM THEGROP CONSISTING OF TITANIUM, ZIRCONIUM, NIOBIUM, HAFNIUM, TANTALUM ANDTUNGSTEN, AND WHEREIN THE CARBIDE PARTICLES ARE UP TO 44 MICRONS INSIZE.
 2. The conductive paint formulation of claim 1 wherein: theconcentration of carbide particles is between 10 and 30% by weight. 3.The conductive paint formulation of claim 2 wherein: the carbideparticles consist essentially of particles within the range of size of 1to 5 microns.
 4. The conductive paint formulation of claim 3 wherein:the concentration of carbide particles is within the range of 10 to 15%by weight.